Ignition systems that work in accordance with the principle of a resonance transducer are described, for example, in German Patent No. 44 09 985 A1 or European Patent No. 0 674 102 A2. They describe systems in which a suitably excited resonant circuit, comprised of the primary winding of the ignition transformer and of a capacitor connected in series thereto, ensures that an a.c. current is produced.
The advantages of an a.c. current ignition can be seen, in particular, in that various spark currents are attainable, and that the spark duration is only limited by the maximum output of the power supply unit. These advantages are derived from the fact that in an a.c. current ignition, the energy is continuously transferred to the spark. In contrast, a known inductive ignition system stores the energy in the coil. In the case of the inductive system, the energy must first be determined as accurately as possible to achieve an adequate voltage supply. However, in conventional inductive ignition systems, as soon as this energy packet is "sent off", the spark energy is fixed and can only be influenced through additional measures.
Generally, known ignition systems, whether they work capacitively or inductively, are engineered for the maximum requirements of the engine. This means that an ignition system of this kind works with the same ignition parameters in all operating points of the engine. The result in such an ignition system that is not adaptable can be unnecessary spark plug wear.
Present-day engines work under widely varying operating conditions, for example, due to the influence of the exhaust-gas recirculation. The need therefore exists for an ignition system which can adapt to various operating conditions.